One known navigation system is the GPS system (Global Positioning System) which presently comprises more than 24 satellites, of which usually a half of them are simultaneously within the sight of a receiver. These satellites transmit e.g. Ephemeris data of the satellite, as well as data on the time of the satellite. A receiver used in positioning normally deduces its position by calculating the propagation time of signals received simultaneously from several satellites belonging to the positioning system to the receiver and calculates the time of transmission (ToT) of the signals. For the positioning, the receiver must typically receive the signal of at least four satellites within sight to compute the position. The other already launched navigation system is the Russian-based GLONASS (Global'naya Navigatsionnaya Sputnikovaya Sistema).
In the future, there will also exist other satellite based navigation systems than GPS and GLONASS. In the Europe the Galileo system is under construction and will be operational within a few years. Space Based Augmentation Systems SBAS (Wide Area Augmentation System WMS, European Geostationary Navigation Overlay Service EGNOS, GPS Aided GEO Augmented Navigation GAGAN) are also being ramped up. Local Area Augmentation Systems LAAS, which uses fixed navigation stations on the ground, are becoming more common. Strictly speaking, the Local Area Augmentation Systems are not actually satellite based navigation systems although the navigation stations are called as “pseudo satellites” or “pseudolites”. The navigation principles applicable with the satellite based systems are also applicable with the Local Area Augmentation Systems. Pseudolite signals can be received with a standard GNSS (Global Navigation Satellite System) receiver. Moreover, Japanese are developing their own GPS/Galileo complementing system called Quasi-Zenith Satellite System QZSS.
The satellite based navigation systems, including systems using pseudo satellites, can collectively be called as Global Navigation Satellite Systems (GNSS). In the future there will probably be positioning receivers which can perform positioning operations using, either simultaneously or alternatively, more than one navigation system. Such hybrid receivers can switch from a first system to a second system if e.g. signal strengths of the first system fall below a certain limit, or if there are not enough visible satellites of the first system, or if the constellation of the visible satellites of the first system is not appropriate for positioning. Simultaneous use of different systems comes into question in challenging conditions, such as urban areas, where there is limited number of satellites in view. In such cases, navigation based on only one system is practically impossible due to the low availability of signals. However, hybrid use of different navigation systems enables navigation in these difficult signal conditions.
Each satellite of the GPS system transmits a ranging signal at a carrier frequency of 1575.42 MHz called L1. This frequency is also indicated with 154f0, where f0=10.23 MHz. Furthermore, the satellites transmit another ranging signal at a carrier frequency of 1227.6 MHz called L2, i.e. 120f0. In the satellite, the modulation of these signals is performed with at least one pseudo random sequence. This pseudo random sequence is different for each satellite. As a result of the modulation, a code-modulated wideband signal is generated. The modulation technique used makes it possible in the receiver to distinguish between the signals transmitted from different satellites, although the carrier frequencies used in the transmission are substantially the same. Doppler effect results in a small (±1 kHz) change in the carrier frequency depending upon the constellation geometry. This modulation technique is called code division multiple access (CDMA). In each satellite, for modulating the L1 signal, the pseudo sequence used is e.g. a so-called C/A code (Coarse/Acquisition code), which is a code from the family of the Gold codes. Each GPS satellite transmits a signal by using an individual C/A code. The codes are formed as a modulo-2 sum of two 1023-bit binary sequences. The first binary sequence G1 is formed with a polynomial X10+X3+1, and the second binary sequence G2 is formed by delaying the polynomial X10+X9+X8+X6+X3+X2+1 in such a way that the delay is different for each satellite. This arrangement makes it possible to produce different C/A codes with an identical code generator. The C/A codes are thus binary codes whose chipping rate in the GPS system is 1.023 MHz. The C/A code comprises 1023 chips, wherein the code epoch is 1 ms. The L1 carrier signal is further modulated with navigation information at a bit rate of 50 bit/s. The navigation information comprises information about the health of the satellite, its orbit, time data, etc.
In the GPS system, satellites transmit navigation messages including Ephemeris data and time data, which are used in the positioning receiver to determine the position of the satellite at a given instant. These Ephemeris data and time data are transmitted in frames which are further divided into subframes. FIG. 6 shows an example of such a frame structure FR. In the GPS system, each frame comprises 1500 bits which are divided into five subframes of 300 bits each. Since the transmission of one bit takes 20 ms, the transmission of each subframe thus takes 6 s, and the whole frame is transmitted in 30 seconds. The subframes are numbered from 1 to 5. In each subframe 1, e.g. time data is transmitted, indicating the moment of transmission of the subframe as well as information about the deviation of the satellite clock with respect to the time in the GPS system.
The subframes 2 and 3 are used for the transmission of Ephemeris data. The subframe 4 contains other system information, such as universal time, coordinated (UTC). The subframe 5 is intended for the transmission of almanac data on all the satellites. The entity of these subframes and frames is called a GPS navigation message which comprises 25 frames, or 125 subframes. The length of the navigation message is thus 12 min 30 s.
In the GPS system, time is measured in seconds from the beginning of a week. In the GPS system, the moment of beginning of a week is midnight between a Saturday and a Sunday. Each subframe to be transmitted contains information on the moment of the GPS week when the subframe was transmitted. Thus, the time data indicates the moment of transmission of a certain bit, i.e. in the GPS system, the moment of transmission of the last bit in the subframe. In the satellites, time is measured with high-precision atomic chronometers. In spite of this, the operation of each satellite is controlled in a control centre for the GPS system (not shown), and e.g. a time comparison is performed to detect chronometric errors in the satellites and to transmit this information to the satellite.
The number of satellites, the orbital parameters of the satellites, the structure of the navigation messages, etc. may be different in different navigation systems. Therefore, the operating parameters of a GPS based positioning receiver may not be applicable in a positioning receiver of another satellite system. On the other hand, at least the design principles of the Galileo has indicated that there will be some similarities between GPS and Galileo in such a way that at least Galileo receiver should be able to utilize GPS satellite signals in positioning.
Positioning devices (or positioning receivers) i.e. devices which have the ability to perform positioning on the basis of signals transmitted in a navigation system can not always receive strong enough signals from the required number of satellites. For example, it may occur that when a three-dimensional positioning should be performed by the device, it can not receive signals from four satellites. This may happen indoors, in urban environments, etc. Methods and systems have been developed for communications networks to enable position in adverse signal conditions. If the communications network only provides navigation model assistance to the receiver, the requirement for a minimum of three signals in two-dimensional positioning, or four signals in three-dimensional positioning does not diminish. However, if the network provides, for instance, barometric assistance, which can be used for altitude determination, then three satellites is enough for three-dimensional positioning. These so called assisted navigation systems utilise other communication systems to transmit information relating to satellites to the positioning devices. Respectively, such positioning devices which have the ability to receive and utilize the assistance data can be called as assisted GNSS receivers, or more generally, assisted positioning devices.
Currently, only assistance data relating to GPS satellites can be provided to assisted GNSS receivers in CDMA (Code Division Multiple Access), GSM (Global System for Mobile communications) and W-CDMA (Wideband Code Division Multiple Access) networks. This assistance data format closely follows the GPS navigation model specified in the GPS-ICD-200 SIS (ICD, Interface Control Document; SIS, Signal-In-Space) specification. This navigation model includes a clock model and an orbit model. To be more precise, the clock model is used to relate the satellite time to the system time, in this case the GPS time. The orbit model is used to calculate the satellite position at a given instant. Both data are essential in satellite navigation.
The availability of the assistance data can greatly affect the positioning receiver performance. In the GPS system, it takes at least 18 seconds (the length of the first three subframes) in good signal conditions for a GPS receiver to extract a copy of the navigation message from the signal broadcasted by a GPS satellite. Therefore, if no valid copy (e.g. from a previous session) of a navigation model is available, it takes at least 18 seconds before the GPS satellite can be used in position calculation. Now, in AGPS receivers (Assisted GPS) a cellular network such as GSM or UMTS (Universal Mobile Telecommunications System) sends to the receiver a copy of the navigation message and, hence, the receiver does not need to extract the data from the satellite broadcast, but can obtain it directly from the cellular network. The time to first fix (TTFF) can be reduced to less than 18 seconds. This reduction in the time to first fix may be crucial in, for instance, when positioning an emergency call. This also improves user experience in various use cases, for example when the user is requesting information of services available nearby the user's current location. These kinds of Location Based Services (LBS) utilize in the request the determined location of the user. Therefore, delays in the determination of the location can delay the response(s) from the LBS to the user.
Moreover, in adverse signal conditions the utilization of the assisted data may be the only option for navigation. This is because a drop in the signal power level may make it impossible for the GNSS receiver to obtain a copy of the navigation message. However, when the navigation data is provided to the receiver from an external source (such as a cellular network), navigation is enabled again. This feature can be important in indoor conditions as well as in urban areas, where signal levels may significantly vary due to buildings and other obstacles, which attenuate satellite signals.
The international patent application publication WO 02/67462 discloses GPS assistance data messages in cellular communications networks and methods for transmitting GPS assistance data in cellular networks.
When a mobile terminal having an assisted positioning receiver requests for assistance data, the network sends the mobile terminal one navigation model for each satellite in the view of the assisted positioning receiver. The format in which the assistance data is sent is specified in various standards. Control Plane solutions include RRLP (Radio Resource Location Services Protocol) in GSM, RRC (Radio Resource Control) in W-CDMA and IS-801.1/IS-801.A in CDMA. Broadcast assistance data information elements are defined in the standard TS 44.035 for GSM. Finally, there are User Plane solutions OMA SUPL 1.0 (Open Mobile Alliance, Secure User Plane for Location) and various proprietary solutions for CDMA networks. The common factor for all these solutions is that they only support GPS.
However, due to the ramp up of Galileo, all the standards shall be modified in the near future in order to achieve Galileo compatibility.
Hence, it is clear that GPS assistance alone will not be adequate in the near future and a new data format must be developed in order to be able to support the new systems.
The problem in providing assistance data for new systems, as well as to GPS, can be reduced to finding a navigation model (clock and orbit model) that can be used to describe all the satellite systems. A straightforward solution is to take the native navigation message format for each of the systems and use this format. However, this would result in various different messages (different message format for each system) which would make the implementation task problematic. Moreover, the native format may also be incompatible with cellular standards. Therefore, the final solution must be such that various different formats are not required.
The challenges in developing a common format include firstly satellite indexing. The satellite index is used to identify the navigation model with a specific satellite. The problem is that every system has its own indexing method.
GPS indexes satellites (SV, Space Vehicle) based on PRN (Pseudo-Random Noise) numbers. The PRN number can be identified with the CDMA spread code used by the satellites.
Galileo uses a 7-bit field (1-128) to identify the satellite. The number can be identified with the PRN code used by the satellite.
GLONASS uses a 5-bit field to characterize satellites. The number can be identified with the satellite position in the orbital planes (this position is called a “slot”). Moreover, in contrast to other systems, GLONASS uses FDMA (Frequency Division Multiple Access) to spread satellite broadcasts in spectrum. It is noted here that there is also a CDMA spread code in use in the GLONASS. There is, therefore, a table that maps the satellite slot number to the broadcast frequency. This map must be included in any assistance data format.
SBAS systems use PRN numbers similar to GPS, but they have an offset of 120. Therefore, the first satellite of the SBAS system has a satellite number of 120.
Since QZSS SIS ICD is not public yet, there is no detailed information on the satellite indexing in the system. However, since the system is a GPS augmentation, the GPS compatible format should at high probability be compatible with QZSS as well.
Pseudolites (LAAS, Local Area Augmentation System) are the most problematic in the indexing sense. There is no standard defined for indexing pseudolites currently. However, the indexing should at least loosely follow the GPS-type indexing, since they use GPS-type PRNs. Therefore, by ensuring that the range of satellite indices is sufficient, it should be possible to describe LAAS transmitters with GPS-type satellite indexing.
The second challenge is the clock model. The clock model for any system is given bytSYSTEM(t)=tSV(t)−[a0+a1·(tSYSTEM(t)−tREFERENCE)+a2·(tSYSTEM(t)−tREFERENCE)2]where tSYSTEM(t) is the system time (for instance, GPS time) at instant t, tSV(t) satellite time at instant t, tREFERENCE is the model reference time and ai (iε{0,1,2}) are the 0th, 1st and 2nd order model coefficients, respectively. Relativistic correction term is not shown in the equation. Since the equation is the same for each system, the only problem in developing the generalized model is to find such bit counts and scale factors that the 1)range of values required by each system is covered and the 2)accuracy (or resolution) requirements for each system are met.
The third problem includes the orbit model. Again, each system has its own format (excluding GPS and Galileo that use the same format). GPS and Galileo use Keplerian orbit parameter set: 6 orbit parameters, 3 linear correction terms as well as 6 harmonic gravitation correction terms. In contrast to GPS and Galileo, GLONASS navigation model only contains information on the satellite position, velocity and acceleration at a given instant. This information can then be used (by solving an initial value problem for the equations of motion) to predict the satellite position at some instant. SBAS utilizes in some sense format similar to GLONASS. The SBAS navigation message includes information on the satellite position, velocity and acceleration in ECEF (Earth-Centered Earth Fixed coordinate system definition) systems at a given instant. This data is used to predict the satellite position by simple extrapolation, which is in contrast with GLONASS, in which equations of motion are integrated in time. Again, since the QZSS ICD is not available yet, the detailed format of the navigation message is not known. However, there are documents that quote the QZSS signal being compatible with either GPS-type ephemeris or SBAS-type broadcast. Hence, ensuring that the new format is compatible with GPS and SBAS, the QZSS orbits may be described using the format of GPS. LAAS require that the orbit model is able to describe objects that are stationary in the ECEF-frame. Also, pseudolites have fairly strict resolution requirements for the position. It is necessary to be able to describe a pseudolite position at a resolution of about 5 mm in some cases.
In addition to these requirements (indexing, clock model and orbit model), the navigation model must include information on model reference time (tREFERENCE in the clock model, similar time stamp is required for the orbit model), model validity period, issue of data (in order to be able differentiate between model data sets), and SV health (indicates whether navigation data from the SV is usable or not).
Needless to say, almost all the systems have their own method of expressing these items. The range and accuracy requirements vary from system to system. Moreover, the current satellite health field requires modification, since in the future GPS (and other systems) do not transmit only one signal, but various signals at different frequencies.
Now, the new assistance data format must be such that all the system specific items as well as parameter range and accuracy requirements are taken into account.
Finally, the problem with current assistance data format is that it only allows for one set of navigation data to be available for a given satellite. This means that when the navigation model is updated, the terminal must be provided with a new set of data. However, already now there are commercial services that provide navigation data that is valid for 5-10 days. The navigation model validity time does not increase, but the service sends multiple sets of navigation data for one satellite. In assisted GNSS this is advantageous, since the user receives all the assistance needed for the next couple of weeks in a single download. The new assistance data format must, therefore, be able to support these long-term orbit fits in current models.
To this date there has been no solution to the problem. This is because the assistance data distribution has been limited to GPS-system and to CDMA-networks.
The current solution in distributing assistance data to the terminals is to obtain navigation model for GPS directly from the satellite broadcasts, modify these data and distribute it to terminals in the network according to various standards in use.